Acoustic waves that encounter a change in acoustic impedance will be at least partially reflected. This presents a problem for efficient and wideband operation of a piezoelectric transducer, since the acoustic impedance of the transducer may differ from the acoustic impedance of the medium into which acoustic wave energy is to be transmitted. For example, the acoustic impedance of a piezoelectric substrate may differ from the acoustic impedance of a human body by a factor of twenty or more.
In order to improve acoustic transmission between piezoelectric transducers and the media through which wave energy is transmitted and received, acoustic impedance matching layers have been employed. Energy reflection can be reduced by utilizing a front matching layer having a thickness of one-quarter of the wavelength of the operating frequency of the piezoelectric substrate and having an acoustic impedance equal to the square root of the product of the acoustic impedances of the substrate and the medium. The efficiency of transmitting acoustic wave energy may be further enhanced by attaching a front matching layer having an acoustic impedance that gradually changes from that of the first piezoelectric substrate to that of the medium of interest, e.g. water or tissue.
A material with an acoustic impedance that is appropriate for a quarter-wavelength matching layer between a conventional transducer and a medium of interest is often not available or may be difficult to synthesize. Moreover, it is often difficult to form a matching layer substance having an acoustic impedance that varies gradually. Candidate materials having appropriate impedances for matching layers are typically not electrically conductive, presenting another problem since an electric field needs to be generated within the piezoelectric material. In addition, such matching layers typically need to be bonded to the transducer, and the selected bonding material may create a layer that tends to interfere with the acoustic pressure wave transmission, especially at ultrasonic frequencies.
Dicing a piezoelectric ceramic and filling the spaces between the diced ceramic with low acoustic impedance epoxy is another known approach to reducing the acoustic impedance of a transducer. As long as the diced elements are small relative to the wavelength of the transmitted waves, the effective acoustic impedance of the transducer is reduced as a function of the volume fraction of the piezoelectric ceramic that is removed. The dicing technique is described in "New Opportunities in Ultrasonic Transducers Emerging from Innovations in Piezoelectric Materials," W. A. Smith, SPIE (Society of Photo-Optical Instrumentation Engineers), Volume 1733 (1992), pages 3-24. The dicing is typically performed by micromachining with fine circular saws. Consequently, there is a limit to the center-to-center distance between cuts. At high frequencies, e.g. 10 MHz, the distances are extremely small and the implementation of the technique is costly.
As an alternative to dicing the piezoelectric substrate, micromachining and then bonding a quarter-wavelength thick matching layer to achieve a desired matching layer acoustic impedance was disclosed by M. I. Haller and B. T. Khuri-Yakub in an article entitled "Micromachined Ultrasonic Materials," in 1991 IEEE Ultrasonics Symposium, pages 403-405. In this technique, etching trenches or holes in silicon may be used to produce high aspect ratio fins or posts in a matching layer that is then bonded to a piezoelectric substrate. However, at high frequencies the layer of bonding material for attaching the matching layer to the piezoelectric substrate potentially interferes with acoustic wave transmission, since the thickness of the bond layer becomes comparable to the thickness of the matching layer.
The various techniques for achieving impedance matching are known, but there are difficulties when operating at high frequencies. The imposed limit may be a result of an unavailability of a suitable material or the result of a necessity of forming a very thin bonding layer that is acoustically transparent at the operating frequency. What is needed is a method of forming a piezoelectric transducer having an impedance matching layer for operation at high frequencies.